Impact of the Metallic Crystalline Structure on the Properties of

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Impact of the Metallic Crystalline Structure on the Properties of Nanocrystals and Their Mesoscopic Assemblies Marie-Paule Pileni* CEA/IRAMIS, CEA Saclay, 91191 Gif-sur-Yvette, France CONSPECTUS: The spontaneous assembly of uniform-sized globular entities into ordered arrays is a universal phenomenon observed for objects with diameters spanning a broad range of length scales. These extend from the atomic scale (10−8 cm), through molecular and macromolecular scales with proteins, synthetic low polymers, and colloidal crystals (∼10−6 cm), to the wavelength of visible light (∼10−5 cm). The associated concepts of sphere packing have had an influence in diverse fields ranging from pure geometrical analysis to architectural models or ideals. Self-assembly of atoms, supramolecules, or nanocrystals into ordered functional superstructures is a universal process and prevalent topic in science. About five billion years ago in the early solar system, highly uniform magnetite particles of a few hundred nanometers in size were assembled in 3D arrays.1 Thirty million years ago, silicate particles with submicrometer size were self-organized in the form of opal.2 Opal is colorless when composed of disordered silicate microparticles whereas it shows specific reflectivity when particles order in arrays. Nowadays, nanocrystals, characterized by a narrow size distribution and coated with alkyl chains to maintain their integrity, self-assemble to form crystallographic orders called supracrystals. Nanocrystals and supracrystals are arrangements of highly ordered atoms and nanocrystals, respectively. The morphologies of nanocrystals, supracrystals, and minerals are similar at various scales from nanometer to millimeter scale.3,4 Such suprastructures, which enable the design of novel materials, are expected to become one of the main driving forces in material research for the 21st century.5,6 Nanocrystals vibrate coherently in a supracrystal as atoms in a nanocrystal. Longitudinal acoustic phonons are detected in supracrystals as with atomic crystals, where longitudinal acoustic phonons propagate through coherent movements of atoms of the lattice out of their equilibrium positions. These vibrational properties show a full analogy with atomic crystals: In supracrystals, atoms are replaced by (uncompressible) nanocrystals and atomic bonds by coating agents (carbon chains), which act like mechanical springs holding together the nanocrystals. Electronic properties of very thick (more than a few micrometers) supracrystals reveal homogeneous conductance with the fingerprint of the isolated nanocrystal. Triangular single crystals formed by heat-induced (50 °C) coalescence of thin supracrystals deposited on a substrate as epitaxial growth of metal particles on a substrate with specific orientation produced by ultrahigh vacuum (UHV). Here we demonstrate that marked changes can occur in the chemical and physical properties of nanocrystals differing by their nanocrystallinity, that is, their crystalline structure. Furthermore, the properties (mechanical, growth processes) of supracrystals also change with the nanocrystallinity of the nanoparticles used as building blocks.

1. INTRODUCTION Advances in the synthesis of colloidal nanocrystals permit one nowadays to exercise such fine control over their chemical composition, size, and shape11−13 that it is possible to selfassemble quasi-spherical10 building blocks with low size and narrow shape distribution into supracrystals. Spherical nanoparticles, assimilated to hard spheres with short-range interactions, self-assemble in various crystalline structures, such as face-centered cubic ( fcc), hexagonal close packing (hcp), and body-centered cubic (bcc) lattices. The crystalline structure of such assemblies markedly depends on several factors, such as vapor pressure, solvent, inclusion of organic molecules (free coating agents, impurities, surfactant), and anisotropic ligand coverage. Supracrystals constitute ideal systems for studying novel chemical and physical properties originating from the collective interactions between nanoparticles. Such a new class of materials exhibits specific properties that are neither those of © 2017 American Chemical Society

the isolated nanoparticles nor those of the corresponding bulk phase.4,7−10,15−18 During the last two decades, a rather large variety of collective properties have been investigated, including collective optical and magnetic properties due to dipolar interactions.11−14 Intrinsic properties due to nanocrystal self-ordering in 3D superlattices were demonstrated in various scientific domains such as crystal growth processes, electronic transport, and vibrational properties.7−10 These preliminary results open up, from both technological and fundamental standpoints, a new research area whereas it currently suffers of an extensive lack of knowledge. Here our purpose is limited to the internal atomic lattice structure of the nanoparticles, which will be hereafter referred to as nanocrystallinity. We demonstrate marked changes in the Received: February 20, 2017 Published: July 20, 2017 1946

DOI: 10.1021/acs.accounts.7b00093 Acc. Chem. Res. 2017, 50, 1946−1955

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Accounts of Chemical Research

Figure 1. TEM images of Co nanoparticles differing by their nanocrystallinities: (a) amorphous nanoparticle, (b) hcp nanocrystals, (c) ε phase nanocrystals, and (d) large domains of fcc nanocrystals.

Figure 2. HRTEM images of various structures obtained by submitting Co nanocrystals differing by their nanocrystallinities to oxygen treatments: core/shell (a), yolk/shell (b), and hollow (c) nanoparticles.

days, the film of nanocrystals grown at the air−solvent interface and the precipitate are composed of single-domain 5 nm Au nanocrystals, while their polycrystalline counterparts remain in solution. These nanocrystals are used as seeds and grown in a solution of Au-oleylamine complex. Its concentration, controlled by the volume of the solution, determines the average diameter of nanocrystals with different nanocrystallinities. The gradual Au input on the seed surface prevents any nucleation of new nanocrystals. The Gaussian size dispersion of both singledomain and polycrystalline Au nanocrystals (3−7%) confirms a homogeneous growth process. With Co nanoparticles, various techniques are used: Mild annealing of their amorphous counterparts synthesized in reverse micelles21 produces single-domain hcp structures (Figure 1a,b). The corresponding ε phase is obtained by a hot injection process (Figure 1c).22 A thermal decomposition23 procedure produces polycrystalline fcc structure with well resolved individual crystalline domains (Figure 1d). The control of the nanoparticle size is obtained by adjusting the surfactant and reactive concentrations, as well as the temperature.21 Nanoscale Kinkerdall effect is related to the interdiffusion processes of elements through nanocrystals.24−26 The relative rate in the inward and outward flows of oxygen and cobalt atoms induces formation of various nanoparticle structures such as solid CoO or core/shell (Figure 2a), yolk/shell (Figure 2b),

chemical and physical properties of nanocrystals and of their assemblies in 3D crystalline structures called supracrystals.

2. NANOCRYSTALLINITY The chemical and physical properties related to the crystalline structure of metal nanoparticles called nanocrystallinity have been poorly investigated up to now. This is due to the difficulty of controlling the internal structure of nanoparticles. Very recently, some progress in the syntheses has been achieved, enabling studies of specific chemical and physical properties. Even though the properties related to nanocrystallinity are limited, we already have some evidence that demonstrates marked changes in some chemical and physical properties related to nanocrystallinity. The data presented here will also allow some controversies to be ruled out.30 The methods developed to control nanocrystallinity markedly depend on the metal used. With Au nanocrystals, a new method19 was developed to selectively produce Au single domain nanocrystals or polycrystalline nanoparticles coated with dodecanthiol in the size range from 5 to 13 nm. The synthesis is based on the production of a mixture of singledomain and polycrystalline 5 nm Au nanocrystals obtained by revisiting the organometallic method developed by Stucky et al.20 First, a colloidal solution of 5 nm Au nanocrystals dispersed in toluene is kept under solvent saturation. After 5 1947


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nanoparticle radius, it is demonstrated that the intrinsic scattering rate is larger for single domain than for polycrystalline nanocrystals. This is attributed to electron scattering on crystal defects. Among the considered morphologies, it was found through discrete dipole approximation (DDA) simulation27 that icosahedral, cuboctahedral, and truncated octahedral particles behave like quasispherical particles whereas the optical response of the decahedral nanoparticles significantly differs from the others. This result reveals the important and often neglected role of crystal twinning in determining the LSPR line width. Here, we have to mention that simulations of the LSPR spectra do not take into account internal defects and consequently cannot describe the experimental data. When the energy of the incident light is in resonance with the energy of the electronic dipolar plasmon, scattering by cluster vibrations is observed. For spherical nanocrystals with sizes larger than ∼1 nm, the cluster vibrations are described by modeling the nanocrystal as a continuum nanosphere with diameter the size of the nanocrystal. Until recently, most of the acoustic vibrational frequencies measured in metallic nanocrystals have been successfully interpreted by modeling the vibrations of these nanoparticles as those of an elastic isotropic homogeneous sphere. The fundamental radial (breathing) mode (l = 0) is the main contribution detected in the dynamical response of quasi-spherical metallic nanocrystals, as measured by pump−probe experiments.28 The time-domain experiments mainly probe their fundamental radial breathing mode (RBM) oscillations, due to isotropy of the thermal excitation.28 The nanocrystallinity of the sample is believed to play a fundamental role, but experiments in this direction are still controversial.30,29,31 Strong variations were found between single-domain and polycrystalline Ag structures and hollow Au nanospheres. The RBM amplitude of oscillations for spherical Au nanocrystals with different nanocrystallinity rapidly decays in time. After 6−7 periods, it vanishes below the sensitivity of the measurement. This decay is due to the RBM intrinsic damping19 caused by energy dissipation. A similar behavior is observed with 7 nm Co nanocrystals.32 The quadrupolar mode (l = 2) is easily observed by lowfrequency Raman scattering (LFRS). The LFRS spectrum (Figure 4) of single-domain Au nanocrystals shows a splitting of the quadrupolar vibrations (l = 2), whereas their multiply twinned counterparts are absent.33 This is a remarkable tool to identify Au single-domain nanocrystals as compared to their polycrystalline counterparts. The calculated vibrational fre-

or hollow (Figure 2c) nanoparticles. Note that the metal structures of the initial Co nanocrystals are maintained in the core/shell and in the yolk/shell structures with CoO as shells. Nanocrystals differing by their sizes and nanocrystallinities are deposited on a TEM grid, subjected to an oxygen flow and heated to 200 °C. After aging for 10 min, the heating system is removed and an argon flow is introduced in order to stop the drastic oxidation process. The final structure depends both on nanocrystallinity and nanoparticle size (Figure 3): (i) With

Figure 3. Size-dependence of Kirkendall effect with different crystallinities of Co nanoparticles.

amorphous nanoparticles, solid CoO nanoparticles are produced for smaller sizes and are converted to core/shell structures for larger ones. (ii) With single-domain nanocrystals differing by their crystalline structure (ε and hcp phases), marked changes in the final structures are observed upon increasing the nanocrystal size. The ε phase favors formation of a hollow structure, whereas a transition from single-domain hollow to multidomain core/shell structure takes place with the hcp phase. Co ε phase nanocrystals is a cubic phase and consequently less compact than the hexagonal structure (hcp). This could explain the change in the final structure. (iii) With polycrystalline fcc nanocrystals, a transition from hollow to yolk/shell structure is observed. Presence of defects in the fcc crystalline structure could explain the slight differences obtained between hcp single domain and fcc polycrystalline phases. From these data, we claim that the nanocrystallinity and the nanoparticle size are key parameters in determining the final structures. This should be taken into account for future applications. For any diameter size (from 5 to 13 nm) and nanocrystallinity (single-domain, polycrystals), the localized surface plasmon resonance (LSPR) spectra of Au nanocrystals coated with dodecanthiol and dispersed in chloroform are characterized by a maximum centered at 527 nm.19 The LSPR band of polycrystalline nanocrystals is characterized by a weaker intensity and larger line width compared to single-domain counterparts having the same average size. From a careful study of the LSPR line width as a function of the inverse of the

Figure 4. Stokes and anti-Stokes LFRS spectra of polycrystalline (red) and single domain (green) nanocrystals of Au nanocrystals (λexc= 532 nm). For clarity, the spectra have been independently scaled and vertically shifted. 1948


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Accounts of Chemical Research quencies of a Au-sphere by using the RUS method agree with the experimental measurements.

3. THE SUPRACRYSTALS Nanocrystals, characterized by a low size distribution, are able to self-assemble in 3D crystalline structure called supracrystals as atoms in crystals. With one component used as building blocks, various crystalline structures ( fcc, bcc, hcp) are produced. Two growth mechanisms can be generated:14,34,35 (1) Heterogeneous growth: film supracrystals are formed by slow solvent evaporation process of the nanocrystal colloidal solution. (2) Homogeneous growth: shaped supracrystals grown in solution are produced. These supracrystals are characterized by regular shapes such as octahedrons, triangles, hexagons, 5-fold symmetric stars, etc. The data presented below are obtained through heterogeneous growth processes. Co nanoparticles differing by their internal structures, either amorphous (Figure 5a) or with two types of single-domain structure, ε phase (Figure 5b) and hcp (Figure 5c)], selfassemble in thin films of a few layers. The high-angle electron diffraction (HAED) patterns exhibit several rings corresponding to electron diffraction by specific planes of atomic lattices with fcc crystalline structure in supracrystals made of either single-domain (Figure 5e,f) or amorphous (Figure 5d) nanocrystals. Note that the presence of well-defined arcs is observed in Figure 5e, whereas they are not well-defined in Figure 5f and have totally disappeared in Figure 5d. Similar arcs in the HAED pattern were previously observed in superlattices of self-assembled nonspherical building blocks,36 especially those consisting of Ag37 and Au38 nanocrystals. Such a feature provides evidence for an average coherent alignment of the atomic lattice planes of nanocrystals to one another within the superlattice. This orientational ordering of the atomic lattices of Co nanocrystals is indicative of their faceted morphology and is presumably favored by large facets in single nanocrystals. These latter facets are indeed expected to exhibit large enough area to induce their face-to-face orientation between neighboring nanocrystals. Comparison of the calculated facet surface of icosahedral, decahedral, cubooctahedral, and truncated octahedral Au nanoparticles confirms this assumption.39 From this, it is concluded that the atomic lattice planes of 7 nm hcp-Co nanocrystals are coherently aligned with those of neighboring nanocrystals. Note that the single-domain features markedly change from one single-domain crystalline structure to another: arcs are distinctly observed for the most intense diffraction rings in single-domain hcp-Co supracrystal (Figure 5e) whereas the arc intensity markedly decreases when the supracrystal film is made of single-domain ε-Co nanocrystals (Figure 5f). This is well evidenced through the first diffraction ring profile of hcpand ε-Co nanocrystals (Figure 5g). It is still observed through the second diffraction ring profile (Figure 5h). The high resolution transmission electronic microscopy (HRTEM) shows that amorphous Co nanoparticles are spherical (Figure 5a), ε-Co nanocrystals are characterized by quasi-spherical shapes (Figure 5b), and hcp Co (Figure 5c) nanocrystals present large facets. Hence the facet/facet interactions between nanocrystals induced both translational and orientational ordering, whereas when the facets are reduced or do not exist, the orientation ordering disappears and only the long-range translational order remains. Note that a behavior similar to that

Figure 5. HRTEM images of 7 nm Co nanocrystals differing by their nanocrystallinities such as amorphous (a), single-domain hcp (b), and ε-phase (c). The corresponding HAED patterns recorded in a thin film (around 5 layers) of close-packed of amorphous (d), single-domain hcp (e), and ε-phase (f) nanocrystals. The corresponding profiles for the first (g) and second (h) diffraction ring of ε phase (black) and hcp (red) 7 nm Co nanocrystals. Supracrystal SEM images of the amorphous (i), hcp (j), and ε phase (k) nanocrystals with corresponding diffraction patterns (insets).

presented here was observed by comparing fcc film supracrystals with single domain and polycrystalline 5 nm Au nanocrystals as building blocks.40 The orientational ordering of nanocrystals described above changes the mechanical properties of the film supracrystals. To prove such a claim, nanoindentation measurements with an atomic force microscope (AFM) were performed against various tip sizes (standard and colloidal probe) and nanoindentor (N-indentor).41 Let us consider around 500 nm-thick supracrystal films produced by slow evaporation of the colloidal solution containing the 7 nm Co nanoparticles coated with oleic acid (C18) and differing by their nanocrystallinities, such as amorphous (Figure 5i), ε (Figure 5j), or hcp (Figure 5k) phases. The Young’s moduli, E, are calculated from force− 1949


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Accounts of Chemical Research displacement (F−δ) curves using either Oliver-Pharr42 or DMT43 models for elastoplastic (i.e., irreversible) and elastic (i.e., reversible) deformations, respectively. Figure 6 shows a

between these various samples characterized by similar nanocrystal size (D), coating agent (C18), and interparticle distance (δpp) is attributed to the crystalline structure of the Co nanocrystals. The correlation between the hierarchical mechanical44 behavior of supracrystals and the nanocrystallinity of their building blocks is attributed to specific facet−facet interactions tuned by the orientational and translational alignment of nanocrystals within the superlattice as shown on Figure 5. The Young’s modulus of the film supracrystals markedly changes with the nanocrystallinity: it was shown that it drops from 550 ± 160 MPa to 25 ± 9 MPa by replacing single domain 5 nm Au nanocrystals coated with dodecanethiol by their polycrystalline counterparts.40 When similar conditions (nanocrystal size, average distance between nanocrystals) are kept and the terminal group of the alkyl chains used as coating agents is replaced with a water-soluble group, the Young’s modulus drops from single-domain to polycrystalline nanocrystals (315 ± 82 and 172 ± 39 MPa, respectively). Note that the drop is smaller for water-soluble than hydrophobic nanocrystals. This is attributed to sterical hindrance.45 By evaporation of a colloidal mixture of single-domain and polycrystal 5 nm Au nanocrystals, a long distance SEM image (Figure 7a) shows the presence of well-defined triangular assemblies (Figure 7b) and of films (Figure 7c). Diffraction patterns indicate that these assemblies are ordered with a fcc structure. The fast Fourier transform (FTT) patterns (insets of Figures 7b,c) confirm the presence of compact hexagonal networks. By low-frequency micro-Raman scattering, a splitting of the quadrupolar peak in the triangular supracrystal region appears (Figure 7d) whereas only one band is observed in the film analysis (Figure 7e). According to data presented above (Figure 4), segregation processes occur: the triangular supracrystals are made of single-domain building blocks whereas the film supracrystals are made of polycrystalline nanocrystals. Hence nanocrystallinity segregation takes place during the solvent evaporation process.46 A similar behavior was observed with water-soluble nanocrystals. However, probably because of weaker particle−particle interaction due to the presence of

Figure 6. Force−distance curves, obtained from indentation process, of amorphous-Co (purple), ε-Co (blue), and hcp-Co (red) nanocrystals.

transition in mechanical properties from hcp-Co supracrystals, the stiffest material, to less stiff ε-Co supracrystals, both showing elastic deformation to a soft amorphous-Co supracrystal material that is plastically deformed. The E values of 7 nm Co supracrystals differing by their nanocrystallinities increase by 1 order of magnitude with improvement of the crystalline symmetry of Co nanocrystals in the following order: Ehcp‑Co > Eε‑Co > Eamorphous‑Co (Table 1). The major change Table 1. For Co Nanocrystals with Different Crystalline Phases, Average Diameter (D), Size Dispertion (σ), Interparticle Distance (δpp), and Young’s Modulus (E) with standard error, ΔE D (nm) σ (%) δpp (nm) E (GPa) ΔE (GPa)

amorphous Co

ε-Co

hcp-Co

6.9 8.7 3.4 0.7 0.4

7.1 7.9 3.4 1.7 0.5

7.2 9.7 3.2 6.6 1.5

Figure 7. (a) Low-magnification SEM image of Au supracrystals; (b, c) SEM images of two areas selected from the first SEM image showing, at higher magnification, truncated-tetrahedral supracrystals (b) and supracrystalline films (c). Insets show FFT patterns deduced from HRTEM images; LFRS spectra of Au nanocrystals in truncated-tetrahedral supracrystals (d) and supracrystalline films (e). 1950


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Figure 8. Various binary structures produced with Ag nanocrystals differing by their diameters.45

charges at the alkyl chain end point, segregation takes place but only supracrystal films are produced.45 Slow evaporation of a colloidal solution of a mixture of nanocrystals differing by their diameters produces a plethora of binary structures, analogous to binary atomic and ionic lattices, such as NaCl, AlB2, MgZn2, Cu3Au, CaB6, or NaZn13.(Figure 8).47−51 Here we limit our study to investigate the influence of the nanocrystallinty on the final structure produced. Let us consider a colloidal solution consisting of a mixture of single-domain 5.6 nm Au and 8.4 nm ε-phase Co nanocrystals coated with oleic acid and dodecanthiol, respectively. At the end of a slow solvent evaporation process, Au nanocrystals concentrate into polyhedral (like truncated tetrahedron, distorted square, or rhombus-like) supracrystals embedded in a supracrystal film of Co nanocrystals (Figure 9a). HRSEM pictures of polyhedral Au nanocrystal superlattice surfaces exhibit complex periodical patterns characteristic of vicinal surfaces.52 The crystallographic planes that build the terraces differ from one polyhedral supracrystal to the other. The major observed vicinal surfaces are based on {111} terraces (Figure 9b). However, a few surfaces are also observed with less compact {100} and {110} terraces that are usually unstable in atomic systems, as shown in Figure 9c,d. Hence, by mixing two single domains, Co (ε phase) and Au nanocrystals, one obtains vicinal surfaces like those produced with atomic materials. By replacing 5.6 nm Au single-domain by its polycrystalline counterpart, keeping the same experimental conditions (size, coating agent, solvent, single-domain 8.4 nm Co ε-phase nanocrystals), one obtains segregation of Co and Au nanocrystals both self-assembled in supracrystals with low energy surfaces, as shown in Figure 9e. These data clearly demonstrate the major role played by nanocrystallinity in determining supracrystal surfaces. Up to now we have not been able to explain the growth mechanism involved in formation of high index surface planes in the supracrystals.

Let us consider now the binary system consisting of 7.2 or 9 nm Co nanoparticles with specific nanocrystallinities (amorphous or hcp) and 4 nm polycrystalline Ag nanocrystals. At room temperature, whatever their nanocrystallinity (amorphous or hcp), the 7 nm Co nanoparticles are superparamagnetic; that is, the characteristic magnetic fluctuation time is below 10 s. The amorphous 9 nm Co nanoparticles are also superparamagnetic, but the single-domain hcp ones are ferromagnetic (fluctuation time up to 10 s). Starting with the 7.2 nm Co nanoparticles with different nanocrystallinities (amorphous and single-domain hcp phase), one obtains an AlB2-type (CoAg2) binary superlattice structure with long-range order and a coherence length up to tens of micrometers. Hence, for both amorphous and hcp 7 nm Co nanoparticles, one observes a preferential orientation of the AlB2 (CoAg2) structure, with the (001) plane parallel to the substrate (Figure 10a,b and insets). The Co nanoparticles self-assemble into hexagonal patterns, and the Ag nanoparticles fill the interstices between the Co nanoparticle layers. Here no difference between amorphous and single-domain Co nanoparticles can be observed. Hence, the nanocrystallinity of the superparamagnetic Co nanoparticles does not play any role in the formation of the final structure. With larger superparamagnetic amorphous Co nanoparticles (9 nm instead of 7.2 nm), keeping the 4 nm Ag nanoparticles, triangle-shaped NaCl-type (CoAg) binary nanocrystal superlattices are produced (Figure 9c). By contrast, using ferromagnetic single-domain (hcp) Co nanocrystals instead of amorphous-phase Co, a dodecagonal quasicrystalline order is obtained, together with loosely packed phases such as the NaZn13-type (CoAg13), AuCu-type (CoAg), and AuCu3-type (CoAg3) structures (Figure 10d). At a higher deposition temperature of 65 °C instead of 25 °C, both amorphous and hcp 9 nm Co nanoparticles are superparamagnetic. At such a temperature, one obtains similar structures with both amorphous and single-domain Co nanoparticles, namely, the 1951


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Figure 9. Chemical maps for Co (ε phase, red) and Au (green) either both single-domain (a) or polycrystalline phase (e). HRSEM image of the {221} nanocrystal superlattice plane vicinal to {111} (b), {35 9 1} nanocrystal superlattice plane vicinal to {100} (c), and {430} nanocrystal superlattice plane vicinal to {110} (d).

4. CONCLUSIONS

CoAg3 (Figure 10e,f) and CoAg2 (inset Figure 10e,f) structures, which are consequently independent of the Co nanocrystallinity. The preceding observations, obtained with Co/ Ag binary system (Table 2), clearly show that53 (i) temperature plays a drastic role in determining the final structure of binary systems, with a binary structure evolving from NaCl to a mixture of CoAg3 and CoAg2, and (ii) the superlattice structure can be governed by magnetic interactions. The magnetic interactions, that is, the dipolar interactions between spherical nanocrystals containing magnetic species, have therefore to be considered as another energetic contribution. From these data, we conclude that a Co/Ag quasicrystalline structure of nanocrystals is induced by magnetic interactions. It needs to be pointed out that strong magnetic interactions are already present in 9 nm hcp Co nanocrystals when they are used to grow one component superlattices at low temperature (25 °C). One observes that a large domain of disordered structure accompanied by small domains of ordered structure are present on the copper grid for 9 nm hcp Co nanocrystals, whereas a large domain of ordered face-centered cubic (fcc) structure is present at the higher temperature of 65 °C. This clearly indicates that the magnetic interactions between nanocrystals can lead to local agglomeration of Co nanocrystals, preventing the formation of densely packed ordered structures with high filling factor.

Very surprisingly, the influence of the crystalline structure of nanoparticles (nanocrystallinity) either dispersed in a solvent or deposited on a substrate, on their ability to self-assemble in 3D superlattices (supracrystals) has been sparsely studied. Due to the difficulty to produce a large amount of nanocrystals differing by their nanocrystallinity (single-domain and polycrystal), very few experimental studies have been performed in this area, whereas there exists some theoretical simulations in the literature. Here we demonstrated the following: (1) The breathing mode of nanocrystals remains unchanged whereas the quadrupolar modes are split in Eg and Tg modes. The most controversial studies deal with the vibrational properties of isolated nanocrystals. The present studies allow these controversies to be resolved. (2) The inward and outward flows of oxygen and cobalt atoms, respectively, markedly change with the nanocrystallinity and with the crystalline structure compactness. From personal communications, we know that presently many devices are proposed to use hollow or core/shell Co-nanoparticles for energy release. The influence of the nanocrystallinity has to be taken into account. 1952


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Figure 10. TEM images of binary nanocrystal superlattices assembled from 4 nm polycrystalline Ag nanocrystals and Co nanocrystals differing by sizes and crystalline structures (nanocrystallinity): (a) 7 nm amorphous phase Co nanoparticles, (b) 7 nm hcp-phase Co nanocrystals, (c,e) 9 nm amorphous phase Co nanoparticles at 25 °C (c) and 65 °C (e); (d, f) 9 nm hcp-phase Co nanocrystals at 25 °C (d) and 65 °C (f). The insets in panels a, b, c, e, and f are close-ups of the corresponding TEM images and the inset in panel d is the FFT pattern of the quasicrystal zone.

(3) We know that the magnetic properties of Co nanocrystals markedly change with the structural phase, that is, with their nanocrystallinity. These nanocrystals selfassemble in 3D superlattices and, according to their nanocrystallinity, induce orientational ordering favored by the facet−facet interactions between nanocrystals, consequently improving the mechanical properties of these supracrystals. (4) Very surprisingly, the mixture of single-domain nanocrystals (Co and Au) can induce the formation of vicinal

surfaces, whereas when one of these two nanocrystals is in polycrystalline form, low energy surfaces are produced. (5) Quasicrystalline structures are induced by changing the nanocrystallinity of Co nanoparticles, due to their transition from superparamagnetic to ferromagnetic regime. Hence, the influence of the nanocrystallinity on the chemical and physical properties of nanoparticles, either isolated or selfassembled in 3D superlattices, markedly change with the nanocrystallinity. In the future, we expect to discover a very 1953


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Research Grant of the European Research Council (ERC 267129).

Table 2. Supracrystal Structures Obtained from Different Size and Crystallinity Co Nanoparticles in Binary Co−Ag Systems As a Function of Deposition Temperaturea nanocrystal diameter crystalline structure magnetic properties (25 °C) crystalline structure (25 °C) quasicrystal

7 nm am super

7 nm hcp super

9 nm am super

Co(Ag)2

Co(Ag)2

CoAg

magnetic properties (65 °C) crystalline structure (65 °C)

super

super

■

9 nm hcp ferro

super

Co(Ag)3 Co(Ag)2 super

Co(Ag)3

Co(Ag)3

(1) Nozawa, J.; Tsukamoto, K.; van Enckevort, W.; Nakamura, T.; Kimura, Y.; Miura, H.; Satoh, H.; Nagashima, K.; Konoto, M. Magnetite 3D colloidal crystals formed in the early solar system 4.6 billion years ago. J. Am. Chem. Soc. 2011, 133, 8782−8785. (2) Sanders, J. V. Colour of Precious Opal. Nature 1964, 204, 1151− 1153. (3) Pileni, M. P. Control of the Size and Shape of Inorganic Nanocrystals at Various Scales from Nano to Macrodomains. J. Phys. Chem. C 2007, 111, 9019−9038. (4) Goubet, N.; Pileni, M. P. Analogy Between Atoms in a Nanocrystal and Nanocrystals in a Supracrystal: Is It Real or Just a Highly Probable Speculation? J. Phys. Chem. Lett. 2011, 2, 1024−1031. (5) Motte, L.; Billoudet, F.; Pileni, M. P. Self assembled monolayer of nanosized particles differing by their sizes. J. Phys. Chem. 1995, 99, 16425−16429. (6) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Self-Organization of CdSe Nanocrystallites into Three-Dimensional Quantum Dot Superlattices. Science 1995, 270, 1335−1338. (7) Courty, A.; Mermet, P. A.; Albouy, P. A.; Duval, E.; Pileni, M. P. Self-organized Ag-nanocrystals in fcc ″supra″ crystals: Vibrational Coherence. Nat. Mater. 2005, 4, 395−398. (8) Lisiecki, L.; Polli, D.; Yan, C.; Soavi, G.; Duval, E.; Cerullo, G.; Pileni, M. P. Coherent Longitudinal Acoustic Phonons in ThreeDimensional Supracrystals of Cobalt Nanocrystals. Nano Lett. 2013, 13, 4914−4919. (9) Yang, P.; Arfaoui, I.; Cren, T.; Goubet, N.; Pileni, M. P. Unexpected electronic properties of micrometer-thick supracrystals of Au nanocrystals. Nano Lett. 2012, 12, 2051−2055. (10) Courty, A.; Henry, I.; Goubet, N.; Pileni, M. P. Large triangular single crystals formed by mild annealing of self-organized silver nanocrystals. Nat. Mater. 2007, 6, 900−907. (11) Zhang, Q.; Xie, J.; Yang, J.; Lee, J. Y. Monodisperse Icosahedral Ag, Au, and Pd Nanoparticles: Size Control Strategy and Superlattice Formation. ACS Nano 2009, 3, 139−148. (12) Seo, D.; Yoo, C. I.; Chung, I. S.; Park, S. M.; Ryu, S.; Song, H. Shape Adjustment between Multiply Twinned and Single-Crystalline Polyhedral Gold Nanocrystals: Decahedra, Icosahedra, and Truncated Tetrahedra. J. Phys. Chem. C 2008, 112, 2469−2475. (13) Pileni, M. P. Supra and Nano crystallinity: Specific properties related to crystal growth mechanisms and nanocrystallinity. Acc. Chem. Res. 2012, 45, 1965−1972. (14) Boles, M. A.; Engel, M.; Talapin, D. V. Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials. Chem. Rev. 2016, 116, 11220−11289. (15) Pileni, M. P. Nanocrystal self-assemblies: fabrication and collective properties. J. Phys. Chem. B 2001, 105, 3358−3371. (16) Pileni, M. P. Self-assembly of inorganic nanocrystals: Fabrication and collective intrinsic properties. Acc. Chem. Res. 2007, 40, 685−693. (17) Nie, Z.; Petukhova, A.; Kumacheva, E. Properties and Emerging Applications of Self-Assembled Structures Made from Inorganic Nanoparticles. Nat. Nanotechnol. 2010, 5, 15−25. (18) Gong, J.; Li, G.; Tang, Z. Self-Assembly of Noble Metal Nanocrystals: Fabrication, Optical Property, and Application. Nano Today 2012, 7, 564−585. (19) Goubet, N.; Tempra, I.; Yang, J.; Soavi, G.; Polli, D.; Cerullo, G.; Pileni, M. P. Size and nanocrystallinity controlled gold nanocrystals: synthesis, electronic and mechanical properties. Nanoscale 2015, 7, 3237−3246. (20) Zheng, N.; Fan, J.; Stucky, G. D. One-Step One-Phase Synthesis of Monodisperse Noble-Metallic Nanoparticles and Their Colloidal Crystals. J. Am. Chem. Soc. 2006, 128, 6550−6551. (21) Yang, Z.; Yang, N.; Yang, J.; Bergström, J.; Pileni, M. P. Control of the oxygen and cobalt atoms diffusion through Co nanoparticles differing by their crystalline structure and size. Adv. Funct. Mater. 2015, 25, 891−897.

“Super” and “ferro” stand for the magnetic state, superparamagnetic and ferromagnetic, respectively, and “am” stands for amorphous.

a

large number of other specific properties linked to the type of nanocrystallinity.

■

REFERENCES

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Corresponding Author

*E-mail: [email protected]. ORCID

Marie-Paule Pileni: 0000-0001-6750-0577 Notes

The author declares no competing financial interest. Biography M. P. Pileni is a Distinguished Professor at University P&M Curie and Senior Researcher in Nuclear and Environmental Center, France. She is a member (1999 to present) and chair (2004−2010) of Institut Universitaire de France, IUF, which favors the development of high quality research and interdisciplinary projects among French universities. She has published more 450 articles with h factor of 73. Her major contributions are (i) understanding the fundamentals of the kinetics and mechanisms in colloidal solutions guiding both the creation of inorganic nanocrystals differing by size, distribution, crystalline structure, and shape and the chemical modification of enzymes, (ii) building up of thermodynamically stable states of selfassemblies, both for surfactant molecules (supraaggregates) and for inorganic nanocrystals (supracrystals), (iii) finding collective optical and magnetic properties induced by dipolar interactions due to the nanocrystal arrangements in 1D, 2D, and 3D superlattices, (iv) discovering chemical and physical intrinsic properties due to the crystalline structure of isolated nanocrystals, (v) discovering different physical (vibrational, magnetic, optical) properties of nanocrystal assemblies depending on the crystalline atomic structure of nanoparticles, (vi) developing conceptual analogies between supracrystals and atomic crystalline structures, and (vii) solubilizing hydrophobic supracrystals in aqueous solution for biomedical (imaging and hyperthermia) applications.

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ACKNOWLEDGMENTS M.P.P. thanks Drs. I. Arfaoui, Y. Con, N. Goubet, S. Mourdikoudis, H. Portales, Y. Wan, J. Wei, N. Yang, and Z. Yang from her group. Thanks are due to Prof. G. Cerullo and Dr. D. Polli from Politecnico di Milano and Dr. C. Deeb from MiNaO-Center. Special thanks are due to Dr. P. Bonville from CEA-Saclay. This research was supported by the Advanced 1954


Article

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